The invention relates to a method of melt spinning a group of multifilament yarns from a polymer melt.
For producing a synthetic spun-bonded nonwoven or for making a synthetic tow for producing staple fibers, it is necessary to spin a group of yarns from a polymer melt. Each of the yarns is formed from a plurality of filaments, which are extruded through nozzle bores. In this process, the group of yarns is withdrawn from the spinning zone by a withdrawal means. After the filaments of the group of yarns are emerged from the nozzle bores, the group of yarns undergoes a cooling in a cooling zone until the filaments are solidified.
In the production of a spun-bonded nonwoven, it is preferred to use air flows, as are disclosed, for example, in DE 35 03 818. In so doing, a coolant is directed substantially radially toward the group of yarns in a cooling shaft downstream of the nozzle bores. Directly downstream of the cooling shaft, a draw shaft is formed. The draw shaft has a configuration in the nature of a venturi nozzle, for generating an accelerated air flow for drawing the group of yarns. To this end, the draw shaft connects to a source of vacuum. In this process, the group of yarns is intensively cooled, so that the withdrawal force that is generated by the drawing does not lead to a tearing of the filaments.
In the case wherein the yarns are spun from an annulary arranged row of nozzle bores, the cooling of the group of yarns occurs likewise by a radially directed coolant flow, as is disclosed, for example, in EP 0 536 497. In this process, the group of filaments is cooled immediately after emerging from the nozzle bores by a coolant flow that is radially directed from the inside outward.
In the known method, the group of yarns undergoes an intensive cooling within the cooling zone. With that, the filaments of the group of yarns receive a crystalline preorientation, which determines the subsequent drawing and, thus, the physical properties of the group of yarns. An increase in the production speed in the known method is thus bound to lead to changed physical properties or to filament breaks in the case of an inadequate cooling.
It is therefore an object of the invention to further develop a method of the initially described kind such that it is possible to spin a group of yarns at higher production speeds with unvarying satisfactory physical properties.
The above and other objects and advantages of the invention are achieved by a method of the described type and which recognizes that the solidification of the filaments in the group of yarns is determined upon their emergence from the nozzle bores to their solidification by two mutually influencing effects. It is known that during the cooling of a polymer melt, same solidifies upon reaching a certain temperature. This process is solely dependent on the temperature, and is here named thermal crystallization. In the melt spinning of a group of yarns, the group is withdrawn from the spinneret. In this process, withdrawal forces act upon the filaments of the group of yarns, which cause a tension induced crystallization in the filaments. Thus, during the melt spinning of a group of yarns, thermal crystallization and tension induced crystallization occur in a superposed manner, and lead together to the solidification of the filaments.
The invention now provides a method, wherein the filaments of the group of yarns are cooled such that it is possible to influence both effects for achieving higher production speeds with unvarying satisfactory physical properties. To this end, the filaments of the group of yarns are initially precooled in the cooling zone, which is here named precooling zone, without a solidification of the polymer melt. Subsequently, the group of yarns is directly advanced into a second cooling zone, which is arranged downstream of the precooling zone and upstream of a withdrawal means, and named hereafter aftercooling zone.
Within the aftercooling zone, the filaments of the group of yarns are further cooled until their solidification by the action of a coolant flow, which is directed into the path of the yarn. This coolant flow has a predetermined flow velocity for influencing the yarn friction. As a result, it is possible to influence the withdrawal tension acting upon the filaments in such a manner that the tension induced crystallization occurs with a delay. Since the filaments of the group of yarns are solidified in the precooling zone substantially only in the external zones, the filaments are unable to take up any noteworthy withdrawal tensions. With that, no significant, tension induced crystallization occurs in the precooling zone, but exclusively a thermally caused crystallization. In this process, the group of yarns may be spun from the nozzle bores of a plurality of spinnerets or one spinneret in a linear line arrangement or in a circular line arrangement.
In a particularly advantageous variant of the method, the coolant flow is accelerated to the predetermined flow velocity in an acceleration zone within the aftercooling zone for purposes of influencing the yarn friction. In this process, the acceleration zone is formed preferably directly upstream of the solidification range of the filaments of the group of yarns. With that, it is possible to influence and control the aftercooling in the aftercooling zone independently of the precooling in the precooling zone. On the other hand, it is ensured that the accelerated coolant flow engages the filaments of the group of yarns in a phase, in which the filaments tolerate an externally engaging air friction, without breaking.
For influencing the withdrawal forces acting upon the group of yarns, a particularly advantageous variant of the method provides that the velocity of the coolant flow upstream of the solidification range is at least equal to or somewhat greater than the advancing speed of the filaments. The flow velocity of the coolant flow differs from the advancing speed of the filaments preferably by a factor 0.3 to 2.
The especially advantageous variant of the method is suited in particular for producing yarns of low, medium, or high deniers at a higher production speed and with uniform physical properties. In so doing, the influencing of the tension induced crystallization is performed under substantially unvarying conditions. The precooling of the filaments in the group of yarns after emerging from the nozzle bores, is adjustable in its cooling effect within the cooling zone such that it is possible to keep the position of the solidification range of the filaments in the group of yarns within the aftercooling zone in a predetermined desired range. Thus, the solidification of the filaments of the group of yarns occurs essentially always in the same place, so as to ensure a uniform treatment of the filaments for influencing the tension induced crystallization.
To influence thermal crystallization, the cooling effects that are caused by the coolant in the precooling zone, should be made variable. In this connection, however, it is necessary that the filaments of the group of yarns already exhibit a certain stability in particular in their outer surface layers before entering the aftercooling zone, so as to withstand the coolant flow in the aftercooling zone without damage.
A particularly advantageous variant for controlling the cooling is given by a further development of the invention, wherein the coolant is tempered before entering the precooling zone. In this instance, the coolant may be heated in its temperature to a value preferably in the range from 20xc2x0 C. to 300xc2x0 C. before entering the precooling zone. To spin, for example, a group of yarns with relatively low deniers, the coolant is preheated to a high temperature, for example, by a heating device. This influences thermal crystallization, which starts directly after the emergence from the nozzle bores, in such a manner that the filaments of the group of yarns are not solidified before entering the aftercooling zone. With that, an advantageous tension treatment is possible by a coolant flow, which is directed parallel to the group of yarns, and which leads to the solidification of the filaments in the group of yarns in the desired range of the aftercooling zone. In the case that a group of yarns with a high filament denier is to be spun, the coolant is adjusted to a lower temperature in the precooling zone, so that thermal crystallization is developed before the entry into the aftercooling zone to such an extent that the filaments of the group of yarns exhibit adequate stability when being engaged by the coolant flow.
For adjusting the cooling in the precooling zone, it is suggested according to another advantageous further development of the invention, that the volume flow of the coolant be varied. To this end, one may use, for example, a blower, which permits controlling the volume flow that is blown into the precooling zone.
The method of the present invention is independent of whether the coolant flow is generated in the aftercooling zone by a suction effect or by a blowing effect. The variant of the method, wherein a suction flow prevails in the aftercooling zone, has the advantage that thermal crystallization in the precooling zone and tension induced crystallization in the aftercooling zone can be influenced essentially independently of each other.
For generating a coolant flow by a blowing action, it is possible to blow the coolant into the precooling zone, and to direct it accordingly into the tension zone, or to blow a coolant that is supplied downstream of the precooling zone, directly into the aftercooling zone.
To obtain in particular in the case of a group of yarns with high filament deniers, an adequate cooling in the aftercooling zone, a variant of the method will be especially advantageous, wherein the coolant flow is generated from the coolant leaving the precooling zone and a coolant that is supplied directly upstream of the inlet to the aftercooling zone. The additionally supplied coolant allows to accomplish that in addition thereto the tension induced crystallization is likewise adjustable within wide ranges, thus permitting a further optimization of the physical properties.
The precooling of the filaments in the precooling zone can likewise occur by an air flow that is blown into the precooling zone, or by an air flow that is sucked into the precooling zone.
Based on its flexibility, the method of the present invention is suitable for melt spinning a group of yarns, which is laid to a spun-bonded nonwoven after the solidification of the filaments. In this process, the group of yarns is withdrawn in a linear line arrangement from the nozzle bores and deposited on a screen belt. Preferably, withdrawal nozzles are used as withdrawal means.
However, the method is also very well suited for combining a group of yarns after solidification of the filaments to a tow, which is deposited in a can for the production of staple fibers. In this process, the group of yarns is spun in a circular line arrangement, preferably from an annular nozzle, and advanced through the precooling and the aftercooling zone. After leaving the aftercooling zone, the group of yarns is combined to the tow. However, the tow could also be cut or torn directly to staple fibers in a subsequent process step, so as to be pressed thereafter to a bale.
Nonetheless, after the solidification of the filaments, it is also possible to divide the group of yarns into a plurality of individual yarns, which are wound to packages.
The group of yarns may be spun from a polymer melt on the basis of polyester, polyamide, or polypropylene.